1. Field of the Invention
The present invention relates to (1) substantially flat surfaced emulsion particles polymerized vinyl monomer having a concavity therein which are useful, for example, as additives in paints and coating agents, such as paper coating agents, and in undercoats of thermal recording materials such as information recording paper and (2) a process for making said particles.
2. Description of the Prior Art
In recent years, various types of particulate polymers have been investigated as additives to coating agents. Most commonly used are uniform and homogeneous emulsion polymerized polystyrene particles having particle diameters of 0.2 to 0.5 .mu.m. For example, Japanese Laid-Open Patent (TOKKAISHO) 59-59741 discloses a process wherein an unsaturated carboxylic acid and a vinyl monomer are copolymerized in the presence of an anionic surfactant and/or a nonionic surfactant to prepare an emulsion of the copolymer particles 90% or more of which have 0.2 to 0.28 .mu.m particle diameter. The emulsion obtained by this method is shown to be used for paper coating or in paints, etc. Employed as an organic pigment for the above uses, however, the emulsion does not provide sufficient hiding power and brightness and only by using a large amount of the particles is an advantage achieved in practical use.
Microvoid-containing organic emulsion particles were introduced for the purpose of solving these problems. For example, U.S. Pat. No. 3,152,280 discloses a process for preparing an aqueous dispersion comprising (1) adding at least one monoethylenically unsaturated monomer to form a sheath polymer into a dispersion of core polymer particles comprising at least 5% copolymerized unsaturated carboxylic acid, (2) emulsion polymerizing, and (3) neutralizing and swelling the core polymer with an aqueous volatile base to form microvoids. When the particles obtained by this process are employed in organic pigment to paints or paper coating compositions, hiding power and brightness are improved as compared with an organic pigment employing uniform and homogeneous type particles. The microvoids, however, are sometimes damaged on compounding or in use. Microvoids are damaged, for example, by freezing, by certain types of solvents used in paint compositions and by heating. Consequently, microvoid-containing particles have limitations in compounding and application conditions which are often encountered.
Flattened emulsion particles, for example, polyethylene particles flattened by mechanically hot pressing the polymers (Funtai to Kogyo p. 33, 9, 1986), are known. Thus-obtained flat particles, however, usually have a particle diameter of 1 .mu.m or more and a broad size range. Therefore it is difficult to efficiently and correctly produce particles which are suitable for each use.
The problems caused by using emulsion particles especially as an organic pigment are discussed hereinabove. The problems arising from the use of emulsion particles in thermal recording materials are described hereinafter.
A thermal recording material is obtained, for example, by dispersing in a binder a colorless or light-colored leuco form dye as a color former and a phenolic compound like bisphenol A, etc., as a developer, applying the resulting dispersion onto a substrate, e.g., paper, and then drying it. Thus obtained thermal recording materials readily develop a color by the heat of a thermal head and a thermal pen and thus can be used to record a picture. Therefore they are used widely in information equipment such as facsimile and thermal printers, etc., used in data communications and computer terminals. They are also used for labels, tickets, and commuter's tickets.
Because the speed of such recording information equipment has been progressing heat-sensitive materials are desired which can be used in such high speed recording equipment.
One method of increasing recording speed is by raising the thermal head temperature to attain better color production. However, the raised temperature of a thermal head tends to cause problems such as build up of foreign matter, sticking, etc. It also shortens the life of the thermal head, which makes this method of increasing recording speed impractical.
When paper is used as a substrate, direct application of a heat-sensitive color developing layer forming fluid to the paper makes it penetrate into the interior of the paper. Therefore, a smaller amount of the heat-sensitive color producing component is present on the paper surface, which lowers the produced color density. Japanese Laid-Open Patent (TOKKAISHO) 55-86789 discloses a method of making a clear picture with dense color by forming an undercoat containing fine synthetic resin particles between a substrate and a heat-sensitive color producing layer to prevent this penetration into the paper. The sensitivity of recording material obtained by this method, however, is not satisfactory for high speed recording.